Dielectric fluids used in electrical distribution and power equipment, including transformers, switching gear and electric cables, perform two important functions. Dielectric fluids act as a dielectric and insulating medium, a cooling medium, and they also reduce the corrosive effects of oxygen and moisture. Analyses of dielectric fluid can provide an indication of the insulating material conditions and thus acts as a diagnostic tool for evaluating the solid insulation condition of the transformer.
There are several specific functional properties characteristic of dielectric fluids. The dielectric breakdown, or dielectric strength, for example, provides an indication of a dielectric fluid's ability to resist electrical breakdown and is measured as the minimum voltage required to cause arcing between two electrodes at a specified gap submerged in the fluid. The impulse dielectric breakdown voltage of a dielectric fluid provides an indication of its ability to resist electrical breakdown under transient voltage stresses such as lightning strikes and power surges. The dissipation factor of a dielectric fluid is a measure of the dielectric losses in the fluid; a low dissipation factor indicates low dielectric loss and a low concentration of soluble, polar contaminants.
Because one function of a dielectric fluid is to carry and dissipate heat, factors that significantly affect the relative ability of the fluid to function as a dielectric coolant include viscosity, specific heat, thermal conductivity, and the coefficient of expansion. The values of these properties, particularly in the range of operating temperatures for the equipment at full rating, must be weighed in the selection of suitable dielectric fluids for specific applications.
An ideal dielectric fluid demonstrates chemical and thermal stability over a long service life of 20-30 years, good electric and thermal properties as described above, low flammability (i.e. high fire and flash points), low viscosity and low pour point, miscibility with existing transformer oils, and is non-corrosive and/or compatible with the electrical equipment material to which it is exposed.
Mineral oil-based dielectric fluids admirably demonstrate the above criteria and thus they have been used extensively throughout the world for over a century in these applications. The volume of mineral oil-based dielectric fluids used in power and distribution transformers worldwide is estimated to be about 30 to 40 billion litres. Increasingly, however, there has been concern that the ideal dielectric fluid should also be biodegradable, non-toxic and renewable so at to exhibit little or no detrimental impact on the environment. Mineral oil-based dielectric fluids are poorly, biodegradable, relatively toxic and have no renewable source. Thus, it is not surprising that alternative dielectric fluids have been sought.
Mono-unsaturated fatty acid-containing oils may be obtained from renewable sources (such as vegetable oil, tallow etc) and thus are attractive candidates for substituting petroleum-based dielectric fluids.
Advantageously, mono-unsaturated fatty acid-containing oils may have higher flash and fire point characteristics than mineral oils, which ensure better safety in operation, handling, storage and transportation of such oils and thus the operational safety of transformers using vegetable oil-based dielectric fluids. The excellent fire safety characteristics of mono-unsaturated fatty acid-containing oil-based dielectric fluids make them ideal candidates for high voltage transformers.
Notwithstanding the above advantages, mono-unsaturated fatty acid-containing oils are susceptible to oxidative degradation, and have a higher pour point, higher dissipation factor, higher acidity number, higher moisture content and significantly higher viscosity compared to mineral oils. Many of these deficiencies can be overcome by subjecting the mono-unsaturated fatty acid-containing oils to purification processes to remove water, acid, and conductive contaminants, and a winterization process to improve the mono-unsaturated fatty acid-containing oil's pour point. Additionally, antioxidants can be added to the purified mono-unsaturated fatty acid-containing oil to enhance its oxidative stability.
Low viscosity of any dielectric fluid is an extremely important parameter as the safe operation of power and distribution transformers highly depend on this parameter. Heat dissipation from hot spots, effective circulation and cooling of transformers, smooth functioning of transformers in high voltage operating conditions is largely controlled by the viscous characteristics of the dielectric fluid.
U.S. Pat. No. 5,949,017 discloses electrical transformers containing electrical insulation fluids comprising high oleic acid oil compositions as an alternative to mineral oil-based dielectric fluids with no/little improvement of the viscous properties of either the base oil or the end products.
U.S. Pat. No. 6,280,659 discloses vegetable seed oil insulating fluids with the improvement of only the low temperature viscous characteristics; i.e. pour point rather than the overall viscous characteristics of the vegetable oil-based dielectric fluid. In fact, the finished dielectric fluid described in U.S. Pat. No. 6,280,659 has a viscosity about 100 cSt measured at 40° C., which is much higher than the typical mineral oil-based dielectric fluid. The improvement of pour point (−18° C.) i.e. low temperature viscous characteristics is not appreciable as the pour point parameters of vegetable oils lies between −15° C. and −25° C.
There is still a significant need for biodegradable dielectric fluids from renewable sources which exhibit good electric and thermal properties, low viscosity, chemical and thermal stability, low flammability, low pour point, miscibility with existing transformer oils and long service life of 20-30 years comparable to existing dielectric fluids based on mineral oils.
The present invention overcomes at least some of the above mentioned disadvantages of known vegetable-oil based dielectric fluids by providing a low viscosity mono-unsaturated fatty acid-containing oil-based dielectric fluid.
It is to be understood that, although prior art use and publications may be referred to herein, such reference does not constitute an admission that any of these form a part of the common general knowledge in the art.